Vulvovaginal candidiasis (VVC), predominantly caused by Candida albicans, is one of the most common types of infectious vaginitis. VVC is responsible for great morbidity among women of reproductive-age and significant burden to the health care system due to rising vaginitis-related health care costs. Though C. albicans exists as a commensal member of the vaginal ecosystem, the specific triggers and mechanisms whereby this polymorphic fungus transitions to its infectious state in the vagina are not well understood. My proposed research will address a critical gap in knowledge related to the importance of the vaginal ecosystem in VVC and the functional interactions that take place within the vagina between the host, the microbiota and the mycobiota. The central hypothesis of this work is that the vaginal microbiota and mycobiota serve as an active ecological barrier to the invasion of Candida spp., however changes in composition and abundance of members in these populations enables the evasion of microbial defense mechanisms and development of VVC. The primary goals of this project are to 1) better understand the role/functions of the vaginal microbiota and mycobiota in VVC by identifying differences in community composition and gene expression before, during and after an active infection and 2) to evaluate molecular factors that lead to the development of VVC. We will leverage a unique set of vaginal samples and metadata collected in a 10-week daily-sampling prospective longitudinal study to explore changes in microbial community structure and function in women who experienced VVC. In Aims 1 & 2, a comprehensive taxonomic survey of the microbiota and mycobiota in vaginal samples will be completed using high-throughput sequencing of 16S rRNA and 18S rRNA genes, respectively. Changes in bacterial and fungal abundance will be measured using validated pan- bacterial and pan-fungal qPCR assays. Metatranscriptomics analysis will be performed on samples selected before, during and after VVC episodes from each woman to identify genes and pathways that are up-regulated during VVC. Taken together, these aims will allow us to identify compositional and functional differences between vaginal communities that are associated with the development of VVC and those that are not. Factors associated with the disruption of the vaginal ecosystem or reestablishment of a healthy equilibrium will also be identified. In the short-term, this work will provide an in-depth molecular understanding of the underlying causes of the development of VVC. In the long-term, the proposed research is destined to break new ground in the field of women's reproductive health and improve the quality of life for women who suffer from VVC by enabling improved diagnostic methods and identifying novel targets for new drug development.